Equalisation has been used since the 1940s for correcting the behaviour of inertial systems. Equalisation is now a standard engineering method that is used routinely wherever the response of a system is not ideal or adequate for the requirement. In this case, an engineer applies a filter with the inverse of the system response placed in series with the system can improve the performance of the combined system. This procedure is called equalisation. Examples of systems that use equalisation range from ultrasound transducers in sonar and to give the equivalent of a sharp pulse for fetal monitoring despite the ringing of the transducer, the accurate control of heavy radar dishes, the control of aircraft of deliberately unstable design such as with forward pointing or upswept wings. Equalisation is used in literally thousands of other applications.
Digital filters implementations of equalisation functions have been commonplace since the early 1960s. In the field of communications, R. E. Lawrence and H. Kaufman describes this process in 1971 in “The Kalman Filter for the Equalisation of a Digital Communications Channel”, IEEE Trans. COM-19, No. 12, pp 1137 to 1141, replacing analogue methods that had been used considerably earlier. The need for equalisation in systems has the major market for digital signal processors (DSP), which is a large industry. These DSP systems are often very complex, performing equalisation which is adaptive and dynamic. However, all these systems only operate when the signal frequency is well below the operating frequency of the DSP.
Pre-emphasis is the simplest form of channel equalisation. It involves placing a fixed filter before, within or after the amplifier, that compensates for the general response of the channel.
It is traditional to classify the type of filter at an early stage. There are many ways to classify or consider a filter. Pre-emphasis may be finite in its impulse response, or infinite. The filter may be a continuous in time filter, or discrete in time, where samples are taken and then combined with various weightings. The filter may be implemented in analogue components, either as a linear or non-linear system, or implemented purely digitally, which can span from pipelined state machines to purely software on personal computer.
However, at the speeds considered by the present invention, the digital signal must be treated as an analogue signal, and the boundaries between the other classifications become blurred.
Prior art solutions to the pre-emphasis problem are limited in either speed or capability. At very high speeds, the prior art applies linear structures such as Rs, Ls and Cs, usually with active devices (amplifiers), to create a filter. These filters are insufficient for the adequate pre-emphasis, or are limited to the cut-off frequency of the amplifiers used in the structures. Digital filters are even more limited in frequency because they cannot work at more than half the speed of the sampling rate due to Nyquist sampling limits.
In the present invention the pre-emphasis is designed to overcome bandwidth limitations that may include limits of skin effects, dissipation factors of the line, package, ESD structures and electronic amplifiers, at a speeds close to the cut off frequency of the amplifiers that comprise the amplifier.
The function of the amplifier is to amplify the power of the signal from the level within the integrated circuit to a level sufficient to drive the output load, in this case a transmission line with termination. The amplifier always comprises a series of stages, with increasing size and drive ability. Each stage typically amplifies the signal by a factor of 2 to 6. Theoretical studies indicate the optimum for minimum propagation time through the amplifier is for each stage to amplifier a load around 3.5 times larger than the previous stage but this depends on the technology, optimisation approach and the frequencies involved. In the case of the present invention, propagation time is less significant than ensuring the maximum bandwidth for the combined amplifier structure.
The larger the gain of an amplifier stage, the lower the bandwidth of the stage. Moreover, when there is insufficient bandwidth of each stage, the distortion of the signal across the amplifier becomes bigger with more stages. This second effect can be considered in general as a reduction in bandwidth the more stages that are used. In 1948, R. Walker and H. Wallman in Vacuum Tube Amplifiers, Chapter 2, McGraw Hill Press, showed for the linear portion of the gate response that if the rise time of a signal is tr, then if there are n stages, then the rise time of the signal through those n stages is the square root of the sum of the squares of the rise time of each stage. The 10% to 90% rise time of a signal is between 0.35 and 0.44 of the period near the cut-off frequency of an amplifier, so using this metric the combination of six or more stages typical of an amplifier can be seen to be serious deleterious to the performance. Finally, phase amplification effects occur when the amplifier transmits a signal higher than the cut-off frequency, which causes extreme of both phase and amplitude distortion.
U.S. Pat. No. 4,559,502 to Huijsing et al. describes a multi-stage operational amplifier with more than two stages, suitable for driving low-ohmic loads. A frequency response similar to that shown in FIG. 7 is obtained by using a so-called nested-miller compensation technique. In a nested-miller compensated operational amplifier, multiple compensation capacitors are connected from the output node to intermediate nodes in-between the individual stages, so that transconductance stages charge the state-variable capacitors by currents proportional to the input signal.
A variation of this technique which overcomes this problem is discussed in U.S. Pat. No. 5,155,447, also to Huijsing et al., and by Fan You et al. (Multistage Amplifier Topologies with Nested Gm—C Compensation, IEEE Journal of Solid-State Circuits, Vol. 32, No. 12, 1997). U.S. Pat. No. 5,485,121 to Huijsing and U.S. Pat. No. 5,854,573 to Chan are based on the same technique to avoid the change of polarity at high frequencies. U.S. Pat. No. 5,486,790 to Huijsing discusses a hybrid-nested-miller compensation technique, whereby the capacitive load of the output terminal is reduced, thus improving the slew-rate/power ratio.
Publication WO 00/03476 20 Jan. 2000 (U.S. Patent Application 2002/0003441, publ. 10 Jan. 2002) describes a linear wide-bandwidth negative-feedback system comprising a high-speed driver and a slower linear controller selectively suppressing the error signal in the system's signal band. However, the operational amplifier described in US 2002/0003441 uses pole splitting capacitors forming a load which cannot be easily adjusted, as impedance elements should not be changed dynamically because that will cause distortion. In other words, the capacitance should be adjusted only during a power-up calibration period, and then attain a constant value, or at most be altered at very rare occasions.
A disadvantage of this topology is that the transconductance stages load the input terminals capacitively. Furthermore, the transconductance stages must be able to handle a large common-mode signal swing. Still another problem called “right-plane-zero problem” is that the amplifier's polarity is changed when the compensation capacitors short-circuit the respective transconductance stages causing a lower unity-gain frequency.
Another limitation of the amplifier as described in WO 00/03476 is that only some of the buffer stages of the main chain have the widened bandwidth as provided by respective feedback auxiliary stages. For example, the 3rd gm buffer stage has a negative feedback gm stage, and as a result, a widened bandwidth, while neither the 2nd, not the 4th gm stages do not have negative feedbacks and thus, have limited bandwidth. This is predetermined by the amplifier topology, where the auxiliary stages are connected between the neighbouring nodes in the main chain of logic stages, as seen, e.g. in FIG. 20 of WO 00/03476. With this arrangement, in case the same feedback is provided for the 4th gm stage, the system in whole will loose stability.
These problems are not addressed by any of the prior art: the conventional solution is simply to slow down the signal, or to fabricate an amplifier in ever increasingly exotic materials such as 3-Si, SiGe, Fully saturated SOI, etc. The present invention takes whatever process and material is used and than achieves the highest speed amplifier, or a close approximation thereto, with inherent pre-emphasis.